1. Field of the Invention
The present invention relates to the analysis of sample fluids and more particularly relates to systems for controlling the introduction of sample fluids into an analyzer.
2. Prior Art
Systems for suppling fluid samples for analysis by equipment, such as chromatographic analyzers, have been proposed by the prior art. Some prior art systems have employed a syringe for introducing a predetermined quantity of sample fluid into the analyzer equipment. Sample fluids to be analyzed were disposed in separate closed sample containers; and successive individual fluid samples were removed from their containers, supplied to the syringe, and injected into the equipment.
It is imperative in most sample analyses that the sample fluid being analyzed be as free as possible from any type of foreign substance. Accordingly, the injection syringe was required to be thoroughly purged of one sample fluid and/or any residual cleansing solvent before a succeeding sample was placed in the syringe. The syringes employed for sample fluid injection were quite delicate because of the extremely small quantities of sample fluid they handled, e.g. quantities of from 5 - 50 microliters. This made manual operation and purging of the syringes both tedious and time consuming. Furthermore, when large numbers of samples were being successively analyzed, a skilled operator was required to attend the equipment and perform the tedious and repetitive tack of purging and filling the syringe.
In order to increase the speed and efficiency of the analysis of multiple fluid samples, mechanized syringe handling systems were proposed. The purpose of such systems was to reduce the amount of operator time required in connection with the analysis procedures and to reduce equipment failures, e.g. the syringe breakage and damage which inevitably resulted from frequent handling.
The mechanized systems generally consisted of a supporting tray for sample containers and an injection syringe manipulating mechanism which functioned to enable removal of sample fluid from individual containers, injection of the fluid into the analyzer and purging of the syringe. The sample container trays were usually actuatable to index successive sample containers to a location from which fluid was transferred to the syringe.
While the prior art mechanized systems were effective in reducing the amount of operator time required to analyze fluid samples, several problems relating to syringe manipulation and purging remained unsolved.
In some proposals the mechanized syringe purging left undesirably large quantities of foreign materials in the samples which were injected into the analyzers. In one type of system, for example, the syringe plunger was mechanically reciprocated during purging to draw in and expel successive charges of solvent and/or sample fluid prior to injection of that sample fluid into the analyzer.
In another type of system, a side arm syringe was employed and purging was accomplished by retracting the syringe plunger beyond the syringe side arm port, after which solvent and/or sample fluid was pumped through the syringe barrel for a predetermined period of time.
Both of these purging procedures, while preferable to manual purging, left undesirably large quantities of foreign material in the sample fluid injected into the analyzers. In particular, it is discovered that volatile fluids created pump cavitation which resulted in the formation of gas bubbles in the purge fluid. This reduced the purging effectiveness.
In still other proposals, sample liquids were subjected to a predetermined differential gas pressure for a predetermined period of time so that the sample liquid was forced through the injection syringe and associated conduits to effect purging. Because sample fluid viscosity varied widely, these systems were subject to extending too much sample fluid during the purging process when low viscosity fluids were employed, and did not expend adequate quantities of fluid for complete purging of highly viscous samples. In circumstances where highly volatile fluid samples were, the partial pressure of the fluid vapor tended to substantially increase the applied pressure differential and the purge volume was thus difficult to accurately control.